1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method for correcting registration deviation caused when an image is formed.
2. Description of the Related Art
In recent years, in order to increase the image formation speed of an electrophotographic color image forming apparatus (laser printer), a large number of color image forming apparatuses of a tandem system have been employed. In the tandem system, the same number of developing devices and photoconductive drums as the number of kinds of color materials (toners) are arranged, and images of different colors are sequentially transferred onto a conveyor belt or a recording sheet, and thus a color image is formed. In this type of tandem system color image forming apparatus, since the images of different colors are formed by image formation portions corresponding to those colors, a plurality of factors that causes registration deviation is present between the images of those colors. Hence, various methods for correcting registration deviation are proposed.
As one of the factors for registration deviation, the unevenness of a lens in a deflection scanning device for deflecting and scanning laser light, the displacement of a position where the lens is attached, the displacement of a position where the deflection scanning device is incorporated into the main body of the image forming apparatus or the like can be considered. When this type of problem occurs, the scanning line of laser light is inclined and curved. The degrees of inclination and curving of the scanning line (hereinafter referred to as a profile) are different from color to color, and this results in registration deviation.
As a method of coping with such registration deviation, Japanese Patent Laid-Open No. 2004-170755 discloses that the sizes of inclination and curving of the scanning line in the deflection scanning device are measured with an optical sensor, bitmap image data is corrected so as to cancel out those sizes, and an image is formed based on the corrected image data. Since, in the method of Japanese Patent Laid-Open No. 2004-170755, the registration deviation is electrically corrected by processing the image data, it is unnecessary to perform mechanical adjustment and a step of adjustment at the time of assembly.
Furthermore, Japanese Patent Laid-Open No. 2008-148291 discloses that data is read from an image memory according to the curving (curvature) of the scanning line, and the registration deviation is corrected without the use of a line buffer. Hence, as compared with the above method disclosed in Japanese Patent Laid-Open No. 2004-170755, the method of Japanese Patent Laid-Open No. 2008-148291 can advantageously cope with the registration deviation at lower cost.
In the method disclosed in Japanese Patent Laid-Open No. 2004-170755, line butters that store the bitmap data of a plurality of rounds of scanning are required. The number of line buffers depends on the width of the curving of the scanning line. For example, when the width of the curving of the scanning line corresponds to N lines of image data, line buffers that can store the image data of the N lines are required. Here, since the value of N varies in each of color image forming apparatuses, the number of line buffers that are mounted needs to correspond to the number of lines which exceeds the maximum value of the variations. When, as described above, a large number of line buffers are provided, the capacity of the memory is increased accordingly, and the size of a circuit for correcting the bitmap image data is also increased, with the result that the cost is increased. Hence, as disclosed in Japanese Patent Laid-Open No. 2008-148291, the method of reading data from the image memory without the use of the line buffers can be considered.
However, in the method disclosed in Japanese Patent Laid-Open No. 2008-148291, it is necessary to perform, according to the amount of curving of the scanning line, access to a memory that has not continuous addresses, for each line. In the access to a memory that has not continuous addresses, as the resolution of an image is higher, a larger number of accesses are required. An example of specific values is as follows when performing correction of registration deviation, on a high-resolution image having a resolution of 2400 dpi, by 1 mm in the sub scanning direction from end to end over the image with respect to the width of main scanning that allows image formation.(1 mm/25.4 mm)×2400 dpi≈95
In other words, it is necessary to perform the access to a memory that has not continuous addresses for the read addresses of the memory 95 times for each line. Since images have higher resolution nowadays, as an image has a higher resolution, the number of times of accesses to a memory that has not continuous addresses is increased. Hence, when an image having a higher resolution is dealt with, the performance of the memory access is likely to be degraded.